1. Field of the Invention
Methods and apparatuses consistent with the present invention relate to a Dual Carrier Modulation (DCM). More particularly, the present invention relates to the DCM and a DCM demapping method for soft decision demodulation.
2. Description of Related Art
DCM is for increasing a frequency diversity gain in a receiver to improve error effectiveness for a high speed data rate by broadening a frequency domain.
A DCM modulation method improves reception performance by gaining frequency diversity by transmitting 4 bits in two subcarriers. As an example, the frequency diversity may be gained by simultaneously transmitting the 4 bits b0, b1, b50 and b51 in subcarrier 0 and subcarrier 50.
FIG. 1 illustrates an example of bit-symbol mapping of a subcarrier with respect to the DCM demodulation method. Specifically, FIG. 1 relates to DCM demodulation with respect to the subcarrier 0 and subcarrier 50, which simultaneously transmits the 4 bits b0, b1, b50 and b51. For convenience, the DCM demodulation and DCM modulation of the subcarrier 0 and subcarrier 50 will be described in the specification, and the bit-symbol mapping of remaining subcarriers is performed in the same way.
Referring to FIG. 1, b0 and b1 among the 4 bits are transmitted to an in-phase component of the two subcarriers, and the b50 and b51 are transmitted to a quadrature-phase component of the two subcarriers. Subsequently, b0 and b1 are recovered as the in-phase component of a received signal in the receiver, and the b50 and b51 are recovered as the quadrature-phase component of the received signal.
FIG. 2 illustrates an example of a DCM demodulator in a receiver.
Referring to FIG. 2, the DCM demodulator includes a Fast Fourier Transform (FFT) unit 210, a channel estimation unit 220, a DCM demapper 230, and a decoding unit 240.
The FFT unit 210 outputs signals y0 and y50 from a received signal of the two subcarriers, i.e., subcarrier 0 and subcarrier 50, which acquire an FFT calculation result of the received signal.
The channel estimation unit 220 outputs channel information {tilde over (h)}0, {tilde over (h)}50, of the two subcarriers.
The DCM demapper 230 performs DCM demodulation using the signals y0 and y50 and the channel information of the two subcarriers in order to output soft decisions {tilde over (h)}0, {tilde over (h)}1, {tilde over (h)}50, {tilde over (h)}51 of the 4 bits in correspondence to the DCM demodulation.
The decoding unit 240 decodes the soft decisions of the 4 bits.
The conventional DCM demapper performs DCM demodulation in the following four ways:
i) Using Zero-Forcing Equalization (ZFE) and Linear Combination (LC) for a ZFE-LC method. The ZEF-LC method is simple to embody, however the reception performance is not good due to a noise amplification effect caused by the ZFE. In this case, the noise amplification effect indicates a phenomenon that a noise comparatively increases while a subcarrier, whose channel gain is less, undergoes equalization.
ii) Using a minimum mean square error equalizer (MMSEE) for a MMSEE-LC method, instead of the ZFE, in order to solve the defect of the ZFE-LC. The MMSEE-LC method may supplement the noise amplification effect by minimizing an error of received signal estimation, however there is a defect in estimating an average power of the noise. Namely, the two methods have problems in that reception performance becomes degraded due to the noise amplification since a basic theory, i.e., soft decisions being generated while all data are transmitted by the two subcarriers, of DCM is ignored.
iii) There is a channel state information (CSI) log-likelihood ratio (LLR) method, which calculates a theoretical log likelihood ratio using an exponential/a log function. The CSI method may generates the most accurate soft decision, however there are problems in that this embodiment is difficult to realize since the exponential/log function are required to be calculated, and there may be a serious error due to quantizing. In this case, the CSI indicates channel state information.
iv) There is a CSI (BAL) method, which can remarkably reduce the noise amplification effect without calculating the theoretical LLR. The method is simple to embody since it may be configured by adding a result of a multiplication of a weighting factor and the ZFE-LC for a balance between channel gains, however there is a problem in that the reception performance comparatively degrades more than the CSI (LLR) method since a soft decision far from the theoretical LLR is generated.
Accordingly, a new DCM demapper, which can generate a soft decision close to the theoretical LLR is required.